The present invention relates to systems and methods for directing a miller and especially systems and methods for imaging and milling dies for defect detection and analysis.
Scanning electron microscopes are known in the art. U.S. Pat. No. 5,659,172 of Wagner describes a method for reliable defect detection using multiple perspective Scanning Electron Microscope (SEM) images. A SEM usually includes an electron gun for generating an electron beam, a SEM lens system for focusing and converging the electron beam, a deflection coil for deflecting the electron beam, a detector for detecting electrons, such as secondary emitted electrons or reflected electrons that are emitted/reflected from an object and a processor that is operative to construct SEM images in response to detection signals provided from the detector. Usually, the electron gun, the SEM lens system and the deflection coil are located within a column, that is commonly referred to as SEM column. The resolution of a SEM and its power consumption are inversely proportional to the distance between the SEM column and the object. This distance is also referred to as a working distance. The detector can also be placed within the SEM column.
Focused ion beam (FIB) systems are known in the art. FIB systems are generally utilized to perform die milling and cross sectioning. The milled or cross sectioned die is usually analyzed by an inspection device, such as a SEM, to detect defects. FIB systems can also be utilized to generate FIB images. FIB systems usually include an ion source for generating an ion beam, a FIB lens system for focusing the ion beam to provide a focused ion beam and an ion beam deflector for deflecting the focused ion beam. A FIB system that is operative to generate a FIB image also has a detector and a processor. Usually, the ion source, the FIB lens system and the ion beam deflector are located within a column, that is commonly referred to as FIB column. The detector can also be placed within the FIB column.
SEM images are generated by irradiating an object with an electron beam, collecting signals resulting from an interaction of the electron beam with at least one portion of the object and processing the collected signals. FIB images are generated in a similar analogues manner, except that the object is irradiated with a focused ion beam.
Systems that include both FIB and SEM systems are known in the art and are referred to as FIB/SEM systems. SEM system allows to inspect a surface of an inspected object, such as a surface of a multi layered die. FIB systems allow for milling the surface and exposing inner layers to inspection. Usually, after the FIB mills the die, the SEM system is utilized to inspect the revealed layers and to further analyze the milled die for detecting defects. A prior art FIB/SEM system is the XL860 DualBeam Workstation of FEI. Such a prior art system 10 is illustrated at FIG. 1. System 10 includes FIB column 12 and SEM column 14. FIB column generated ion beam 15 and SEM column 14 generates electron beam 13. System 10 further has stage 18, detectors 26 and 28 and at least one processor (not shown) coupled to detectors 26 and 28 and being operative to generate images. Stage 18 supports the object, such a wafer 20. Detectors 26 and 28 receive electrons, such as secondary emitted electrons and reflected electrons, emitted or reflected from wafer 20 in response to an irradiation of wafer 20 by electron beam 13 or ion beam 15, and provide detection signals to the processor.
FIB column 12 is angularly displaced from SEM column 14 at a predefined angle and is located substantially adjacent to SEM column 14. This arrangement forces both columns to be placed in a relatively large working distance from wafer 20. For example, at XL860 DualBeam Workstation the working distance of the SEM column is 5 mm and the working distance of FIB column 12 is 16.5 mm.
This arrangement also limits the width of the columns and further reduces the performances of at least one of SEM column 14 and FIB column 12.
Ideally, during use of the system for defect detection and analysis, electron beam 13 and focused ion beam 15 are aimed to the same point on an object that is evaluated. Because of the angular displacement between SEM column 14 and FIB column 12 changes in the vertical displacement, caused by changes in the object surface or mechanical inaccuracies of a stage that supports and moves the die, between the surface of wafer 20 and either one of the columns must be compensated by a calibration step. The calibration is usually done manually and is time-consuming. The need for the calibration process is illustrated at FIGS. 2 and 3. A change in the vertical displacement (H1 of FIG. 2 versus H2 of FIG. 3) between surface 21 of wafer 20 and SEM column 14 (and accordingly also between surface 21 and FIB column 12) causes ion beam 13 to irradiate point 27 on surface 20 while electron beam 15 irradiates point 17. In order to have both beams irradiate the same point one of the beams must be slightly deflected.
There is a need to provide an efficient system and method for directing a miller. There is a need to provide a system that efficiently combines the capabilities of a scanning electron microscope and of a focused ion beam generator. There is a need to provide a system and method that allows placement of an object at a small working distance from a scanning electron microscope and at a small working distance from a focused ion beam generator.
The invention provides a method for directing a miller, the system including: a first imager, for locating a landmark on an die; a stage, for moving the die from a first location in which the object is accessible to the first imager to a second location in which the die is accessible to a miller; and a second imager, for directing a miller to mill the die at the desired location.
The second imager can be operative to locate the landmark and to direct the miller to mill at the desired location in response to landmark information and displacement information. The second imager can be operative to generate at least one image of at least one portion of the die and to locate the landmark in response to an analysis of the at least one image of the at least one portion. The second imager can also be operative to generate at least one image of at least one portion of the die and to locate the landmark in response to a comparison between a first image that includes the landmark and the at least one image of the at least one portion, the first image being generated by the first imager.
The first imager can include a scanning electron microscope. The scanning electron microscope includes a SEM column, at least one detector and at least one processor operable to generate SEM images of the die.
The second imager conveniently is also capable of milling the object. A focused ion beam miller and imager can be utilized for imaging and milling. The focused ion beam miller and imager can include a focused ion beam column, for generating and controlling a focused ion beam, at least one detector and a processor, the processor being operable to generate focused ion beam images of the die, to locate the landmark and to direct the miller.
The second imager can include a scanning electron microscope. The scanning electron microscope can include a SEM column, at least one detector and at least one processor operable to generate SEM images of the die.
Conveniently, the first imager and the second miller are spaced apart. The distance between the first imager and the miller exceeds 10 mm. The stage is operative to place the object at a small working distance from the miller and at a small working distance from the first imager. The sum of the small working distance from the miller and the small working distance from the first imager preferably does not exceed 15 mm but can also be limited to 5 mm or even less.
The invention provides a system for directing a miller that is operative to execute a step of angle compensation, for correcting differences between images generated by the first imager and the second imager, the differences were introduced because the images were generated from different angles.
The invention provides a system for directing a miller that is operative to execute a step of imaging process variation compensation, for correcting differences between images generated by the first imager and the second imager, the differences were introduced because the images are generated by distinct image generation processes.
The invention provides a system for directing a miller that is operative to execute a step of determining a landmark group of pixels of the image generated by the first imager and to execute a correlating step for determining a correlation between the landmark group of pixels and pixels of an image generated by the second imager.
The invention provides a system for directing a miller in which the first imager is operable to generate a landmark. The landmark can be generated by adding material to an object or by milling the object. The landmark can be generated by an interaction between an electron beam and an additional material, such as a contamination or an injected material.
The invention provides a system for directing a miller wherein the first imager is operative to generate a landmark. The first imager can generate a landmark by either adding particles to the object or by milling the object. Conveniently, the first imager is a scanning electron microscope that is capable of generating the landmark. A gas injection unit may provide gas to enhance the landmark generation process.
The invention provides a system for defect analysis that includes a scanning electron microscope, for generating SEM images and locating a landmark on an object; a focused ion beam miller and imager, for locating the landmark on the object and milling the object at a desired location; and a stage, for moving the inspected from a first location in which the object is accessible to the scanning electron microscope to a second location in which the object is accessible to a focused ion beam miller and imager.
The invention provides a system for defect analysis wherein the focused ion beam miller and imager is operative to (A) locate the landmark and to mill at the desired location in response to landmark information and displacement information, (B) generate at least one image of at least one portion of the object and to locate the landmark in response to an analysis of the at least one image of the at least one portion, or (C) generate at least one image of at least one portion of the object and to locate the landmark in response to a comparison between a first image that includes the landmark and the at least one image of the at least one portion, the first image being generated by the scanning electron microscope.
The invention provides a system for defect analysis wherein the scanning electron microscope includes a SEM column, and the focused ion beam miller and imager includes a FIB column. The FIB column and the SEM column are spaced apart. The distance between the FIB column and the SEM column exceeds 10 mm. The distance is measured between the aperture of the SEM column and the aperture of the FIB column.
The invention provides a system for defect analysis wherein the stage is operative to place the object at a small working distance from the FIB column and at a small working distance from the SEM column. Conveniently, the sum of the small working distance from the FIB column and the small working distance from the SEM column does not exceed 15 mm, but can also not exceed 5 mm.
The invention provides a system for defect analysis wherein the system is operative to execute at least one of the following steps: (a) a step of angle compensation, for correcting differences between images generated by the scanning electron microscope and the focused ion beam miller and imager, the differences were introduced because the images were generated from different angles; (b) a step of imaging process variation compensation, for correcting differences between images generated by the scanning electron microscope and the focused ion beam miller and imager, the differences were introduced because the images are generated by distinct image generation processes; or (c) a step of determining a landmark group of pixels of the image generated by the scanning electron microscope and to execute a correlating step for determining a correlation between the landmark group of pixels and pixels of an image generated by the focused ion beam miller and imager.
The invention provides a system for defect analysis that includes a gas injection unit for providing a gas that interacts with the electron beam to generate the landmark. The landmark being generated by milling or deposition.
The invention provides a method for directing a miller, the method including the steps of: determining a desired location in which an object is to be milled; detecting a landmark by a first imager; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; and directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark.
The invention provides a for directing a miller, the method including the steps of: determining a desired location in which an object is to be milled; searching for a landmark within a predefined area that includes the desired location; generating a landmark if a landmark is not detected within the predefined area; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; and directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark.
The invention provides a method for milling an object, the method including the steps of: determining a desired location in which an object is to be milled; searching for a landmark within a predefined area that includes the desired location; generating a landmark if a landmark is not detected within the predefined area; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark; and milling the object at the desired location.
The invention provides a method for milling an object, the method including the steps of: determining a desired location in which an object is to be milled; detecting a landmark by a first imager; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark; and milling the object.
The invention provides a method for inspecting an object, the method including the steps of: determining a desired location in which an object is to be milled; detecting a landmark by a first imager; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark; milling the object to provide a milled object; moving the milled object from the second location to the first location; and generating information reflecting the milled object.
The invention provides a method for inspecting an object, the method including the steps of determining a desired location in which an object is to be milled; searching for a landmark within a predefined area that includes the desired location; generating a landmark if a landmark is not detected within the predefined area; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller; locating the landmark; directing the miller to the desired location in response to landmark information and displacement information being generated by the first imager in response to the detection of the landmark; milling the object to provide a milled object; moving the milled object from the second location to the first location; and generating information reflecting the milled object.
The invention provides a method for milling an object, the method including the steps of: determining a desired location in which an object is to be milled; searching for a landmark within a predefined area that includes the desired location; moving the object from a first location in which the object is accessible to the first imager to a second location in which the object is accessible to the miller, if the landmark was not detected within the predefined area; generating a landmark by milling the object and generating landmark information indicative of the landmark; moving the object from the second location to the first location; locating the landmark and generating displacement information; moving the object from the first location to the second location; and detecting the landmark and directing the miller to the desired location in response to the displacement information.
The step of searching for a landmark includes a step of generating images of a portion of the object, by a first imager. The step of generating the images starts by generating a first image of an area that includes the desired location. If the desired location and the landmark are included within that first image the step of searching for an image ends. Else, at least one additional image of at least another portion of the object is generated and analyzed until a landmark is located. It is noted that the landmark can be located outside of an area that is defined by expected inaccuracies in the movement of the object. A landmark can include any shape that is unique within an inaccuracy area that includes the desired location and within an inaccuracy area that includes the landmark. The size and shape of the inaccuracy area are responsive to the accuracy limitation of a stage or other supporting means that supports and moves the object.
According to an aspect of the invention the searching can utilize portion information indicative of the expected image of an at least one portion of the object. The portion information can be driven from previously generated SEM images, from CAD design information and the like.
The invention provides a method for directing a miller, the method involving generating landmark information indicative of the landmark and displacement information reflecting a displacement between the landmark and the desired location. The landmark information can include visual and/or textual information. The visual information can include information representative of an image of the landmark, the landmark and its vicinity, a portion of the image that includes the landmark or the whole SEM image that includes the landmark. The landmark information also defines the location of the landmark. The textual information can include text that describes the landmark such that it can be identified by a second imager. It is noted that the displacement information can also include textual and/or visual information. It is noted that the landmark information and displacement information are stored or transmitted such that they may be retrieved during later steps of the process.
According to an aspect of the invention, the method can include a step of generating desired location information. The desired location information can be utilized for verifying that the miller is directed to the desired location. The desired location information can also include visual and/or textual information.
The step of locating the landmark may involve generating an image and comparing the image to the landmark information, and especially to at least one image generated by the first imager. The step of locating the landmark may include generating images of a portion of the object, starting at a first image of at least a portion of the inaccuracy area that includes the landmark. These images are generated by the second imager. If the first image generated by the second imager includes the landmark then the step of searching the landmark ends. Else, else additional images of portions of the inaccuracy area are taken and analyzed until the landmark is located.
It is noted that the step of locating the landmark can include at least one of the following steps: (A) A step of angle compensation, for correcting differences between the SEM image and the FIB image, the differences were introduced because the images were generated from different angles. The step can include applying a linear transformation on the pixels of at least one image to xe2x80x9ctiltxe2x80x9d the image. (B) A step of imaging process variation compensation, for correcting differences between the SEM image and the FIB image, the differences were introduced because the FIB image was generated by a FIB system, while the SEM image was generated by a SEM system. This step may include (B.1) generating a first edge image indicative of edges within the SEM image, (B.2) generating a second edge image indicative of edges within the FIB image. An image is usually represented by an array or a matrix of pixels. It is noted that generating an edge image from an image is known in the art and can be implemented by various edge enhancement techniques, such as applying a Canny filter on the pixels of each image. Canny filters are known in the art. A Canny filter can be described as a convolution of a Gaussian operator and a step operator. A Gaussian operator is a set of integers that approximate the profile of a Gaussian function along any row, column or diagonal. An edge image can also generated by applying a gradient operator on the pixels of each image. The application of the gradient operator can be preceded by smoothing the picture. The smoothing step limits the amplification of noise.
The process also includes a step of detecting a location of a landmark in the an image generated by the first imager and an image generated by a second imager. Once the landmark is located within both images the miller is directed to the desired location in response to a displacement between the landmark and the desired location. It is noted that if the landmark is not detected in an image generated by the second imager, a step of searching for the landmark is initiated. The step of detecting a location of a landmark can include the following steps: (a) Determining a landmark group of pixels of the image generated by the first imager. The landmark group of pixels are pixels of at least a portion of the landmark but can also include pixels of the vicinity of the landmark. (b) Correlation step in which a correlation between that landmark group and group of pixels within the image generated by a second imager is calculated to provide a correlation coefficient. Conveniently, the image generated by the second imager is scanned to find the group that is most correlated with the landmark group. It the correlation coefficient exceeds a predefined value then the landmark is located. If the correlation coefficient is below the predefined threshold, it is assumed the image does not include the landmark, and another image of a different portion of the object is taken by second imager. The other image is then processed to locate the landmark.
The step of generating a landmark can include implementing various milling, etching, depositing techniques. The landmark can be generated by a scanning electron microscope. The scanning electron microscope generates an electron beam that interacts with the object to generate the landmark. The electron beam can also interact with additional material such as to enhance or speed up the generation of the landmark.
The characteristics of the landmark, such as height or depth can be controlled either automatically or manually to assure that the landmark can be located during the step of detecting the landmark.